EP3898035A1

EP3898035A1 - Method for producing a machining segment for the dry machining of concrete materials

Info

Publication number: EP3898035A1
Application number: EP19820805.0A
Authority: EP
Inventors: Marcel Sonderegger; Cliff Toldo
Original assignee: Hilti AG
Current assignee: Hilti AG
Priority date: 2018-12-21
Filing date: 2019-12-19
Publication date: 2021-10-27
Also published as: US12023737B2; KR20210105347A; US20220055110A1; WO2020127623A1; EP3670037A1

Abstract

A method for producing a machining segment (41) for the dry machining of concrete materials. The machining segment (41) is produced in a three-step process: In a first step a green body is produced from a first matrix material (44) and first hard material particles (45), in a second step the green body is compressed under the effect of pressure to form a pressed product and in a third step the pressed product is further processed to form the machining segment (41).

Description

Process for producing a processing segment for the dry processing of concrete materials

Technical field

The present invention relates to a method for producing a machining segment according to the preamble of claim 1.

State of the art

Machining tools, such as core bits, saw blades, abrasive discs and abrasive chains, include machining segments that are attached to a tubular, disc or ring-shaped base body, the machining segments being connected to the base body by welding, soldering or gluing. Depending on the machining method of the machining tool, machining segments that are used for core drilling are considered as drilling segments, machining segments that are used for sawing, saw segments, machining segments that are used for ablation, removal segments and machining segments that are used for abrasive cutting as abrasive cutting segments designated.

Machining tools that are designed as a core bit, saw blade, abrasive disc or abrasive chain and are intended for wet machining of concrete materials are only suitable to a limited extent for dry machining of concrete materials. When wet machining concrete materials, an abrasive concrete sludge is created that supports the machining process and leads to self-sharpening of the machining segments during machining. The matrix material is removed by the abrasive drilling mud and new hard material particles are exposed. When dry machining concrete materials, no abrasive drilling mud can form that can support the machining process. The hard material particles quickly become dull and the processing rate drops. Due to the lack of concrete sludge, the matrix material wears too slowly and deeper-lying hard material particles cannot be exposed. In known machining tools for wet machining, the matrix material and the hard material particles have similar wear rates. Machining segments for core bits, saw blades, abrasive discs and abrasive chains are made from a matrix material and hard material particles, whereby the hard material particles can be statistically distributed or arranged according to a defined particle pattern in the matrix material. In the case of machining segments with statistically distributed hard material particles, the matrix material and the hard material particles are mixed, the mixture is poured into a suitable tool shape and further processed to form the machining segment. In the case of machining segments with set hard material particles, a green compact is built up in layers from matrix material, in which the hard material particles are placed at defined positions.

Presentation of the invention

The object of the present invention is to develop a method for producing a machining segment with which machining segments can be produced which are suitable for the dry machining of concrete materials. The processing segment for dry processing concrete materials should have a high processing rate and the longest possible service life.

In the method mentioned at the outset, this object is achieved according to the invention by the features of independent claim 1. Advantageous further developments are specified in the dependent claims.

The method for producing a machining segment for a machining tool, the machining segment being connected with an underside to a base body of the machining tool, is characterized in accordance with the invention in that when compacting the green compact, the second press stamp is used, which has depressions in a press surface, whereby the arrangement of the depressions corresponds to the defined particle pattern of the first hard particles. By using a second press ram, which has an arrangement of depressions for the first hard material particles in the pressing surface, the green compact can be compacted to form a compact with a protrusion of the first hard material particles relative to the first matrix material on the upper side. The depressions which correspond to the defined particle pattern of the first hard material particles are required in order to produce the protrusion of the first hard material particles on the upper side during pressing.

Machining segments that are produced using the method according to the invention are produced in a three-stage process: in a first stage, a green compact is built up from the first matrix material and the first hard material particles, the first hard material particles being placed in the first matrix material according to a defined particle pattern are, in a second stage the green compact is compressed under pressure between a first press die, which forms the underside of the processing segment, and a second press die, which forms an upper side of the processing segment opposite the underside, to a compact and is in a third stage the compact is further processed under the influence of temperature or by infiltration to the processing segment.

The method according to the invention enables the production of machining segments with a protrusion of the first hard material particles compared to the first matrix material, where the protrusion of at least one first hard material particle compared to the first matrix material is greater than 400 mhi. Machining segments in which at least one of the first hard material particles has an overhang of more than 400 mhi compared to the first matrix material are suitable for the dry machining of concrete materials. The larger the protrusion of the first hard material particles, the higher the machining rate that can be achieved with the machining tool.

In a first preferred variant, a cover layer of the first matrix material is applied after the placement of the first hard material particles. By applying the top layer of the first matrix material, the first hard material particles can be embedded deep enough in the first matrix material to ensure in the finished machining segment that the first hard material particles are sufficiently fastened and that the first hard material particles do not break out. The height of the top layer of the first matrix material is adapted to the requirements. If the top layer completely embeds the first hard material particles, a strong compression with the second press ram is required in order to produce a protrusion of more than 400 mhi of the first hard material particles on the upper side compared to the first matrix material. If the top layer does not completely embed the first hard material particles, the green compact already has a protrusion of the first hard material particles compared to the first matrix material, but the wear of the second press die is greater than with completely embedded first hard material particles.

In a second preferred variant, after the first hard material particles have been placed, a top layer of a second matrix material is applied, the second matrix material being different from the first matrix material. Applying a top layer of a second matrix material that is different from the first matrix material offers the possibility of using matrix materials with different wear properties. When the cover layer completely embeds the first hard material particles, the finished processing segment has a protective layer on the top. This protective layer is removed by sharpening the machining segments or during machining. In both cases it is beneficial to have one second matrix material with a high wear rate so that the first hard material particles can be released quickly or easily.

In a third preferred variant, after the placement of the first hard material particles, a first cover layer of the first matrix material and a second cover layer of a second matrix material are applied, the second matrix material being different from the first matrix material. The first cover layer of the first matrix material ensures that the first hard material particles are embedded sufficiently deep in the first matrix material, and the second cover layer of the second matrix material can serve as a protective layer for the second press stamp. This protective layer is removed by sharpening the machining segments or during machining. In both cases, it is advantageous to use a second matrix material with a high wear rate, so that the first hard material particles can be released quickly or easily.

In a further development of the method, first hard material particles are used which are enveloped by a shell material, the shell material corresponding to the first matrix material. The use of coated first hard material particles has the advantage that the first hard material particles do not come into direct contact with the second press die and the wear of the second press die can be reduced.

In an alternative development of the method, first hard material particles are used which are enveloped by a shell material, the shell material being different from the first matrix material. The use of coated first hard material particles has the advantage that the first hard material particles do not come into direct contact with the second press die and the wear of the second press die can be reduced. When using a shell material that is different from the first matrix material, matrix materials with different wear properties can be used. The shell material serves to protect the second press ram during compaction and should be able to be removed as quickly as possible from the finished processing segment in order to expose the first hard material particles that process the concrete material.

In a further development, second hard material particles are mixed in with the first matrix material, an average particle diameter of the second hard material particles being smaller than an average particle diameter of the first hard material particles. Depending on the wear properties of the first matrix material, increased wear of the first matrix material on the side surfaces of the machining segment can occur during the machining of a substrate with the machining tool due to friction with the substrate. This wear can be reduced by the second hard material particles. The second hard material particles can be statistically distributed particles of the first material. rix material are added or the second hard material particles are placed according to a defined second particle pattern in the first matrix material. The second hard material particles are placed in particular in the area of the side surfaces of the machining segment.

Embodiments

Embodiments of the invention are described below with reference to the drawing. This is not necessarily to show the exemplary embodiments to scale, rather the drawing, where useful for the explanation, is carried out in a schematic and / or slightly distorted form. It should be taken into account that various modifications and changes regarding the shape and detail of an embodiment can be made without departing from the general idea of the invention. The general idea of the invention is not limited to the exact form or the detail of the preferred embodiment shown and described in the following, or is limited to an object which would be restricted compared to the object claimed in the claims. For given measurement ranges, values within the limits mentioned should also be disclosed as limit values and can be used and claimed as required. For the sake of simplicity, the same reference numerals are used below for identical or similar parts or parts with identical or similar functions.

Show it:

FIGN. 1A, B two variants of a machining tool designed as a core bit;

FIGN. 2A, B two variants of a machining tool designed as a saw blade;

FIG. 3 shows a processing tool designed as a removal disk;

FIG. 4 a processing tool designed as a cut-off chain;

FIGN. 5A-C show a machining segment in a three-dimensional representation (FIG. 5A), in a view on an upper side (FIG. 5B) and in a view on a side surface (FIG. 5C);

FIG. 6 the production of the machining segment of FIGN. 5A-C according to the method according to the invention, a green body being produced in a first stage and the green body being compressed into a compact in a second stage; and FIGN. 7A-C some tool components that are used in the manufacture of the machining segment of FIG. 5A-C can be used.

FIGN. 1A, B show two variants of a machining tool designed as a core bit 10A, 10B. The in FIG. 1A, core drill bit 10A is hereinafter referred to as the first core drill bit and the one shown in FIG. 1B shown core drill bit 10B referred to as the second core drill bit, in addition, the first and second core drill bit 10A, 10B are summarized under the term "core drill bit".

The first core drill bit 10A comprises a plurality of machining segments 11A, a tubular body 12A and a tool holder 13A. The machining segments 1 1A, which are used for core drilling, are also referred to as drilling segments and the tubular base body 12A is also referred to as a drilling shaft. The drill segments 11A are firmly connected to the drill shaft 12A, for example by screwing, gluing, soldering or welding.

The second core drill bit 10B comprises an annular machining segment 11B, a tubular base body 12B and a tool holder 13B. The ring-shaped machining segment 11 B, which is used for core drilling, is also referred to as a drilling ring and the tubular base body 12B is also referred to as a drilling shaft. The drill ring 11 B is firmly connected to the drill shaft 12B, for example by screwing, gluing, soldering or welding.

The core drill bit 10A, 10B is connected via the tool holder 13A, 13B to a core drilling device and is driven by the core drilling device in a direction of rotation 14 about an axis of rotation 15 in the drilling operation. During the rotation of the core drill bit 10A, 10B about the axis of rotation 15, the core drill bit 10A, 10B is moved along a feed direction 16 into a workpiece to be machined, the feed direction 16 running parallel to the axis of rotation 15. The core drill bit 10A, 10B produces a drill core and a borehole in the workpiece to be machined.

The drill shaft 12A, 12B is in the embodiment of FIGN. 1A, B are formed in one piece and the drill segments 11A or the drill ring 11 B are fixedly connected to the drill shaft 12A, 12B. Alternatively, the drill shaft 12A, 12B can be formed in two parts from a first drill shaft section and a second drill shaft section, the drill segments 1 1A or the drill ring 11B being firmly connected to the first drill shaft section and the tool holder 13A, 13B firmly connected to the second drill shaft section is. The first and second drill shaft sections are connected to one another via a releasable connecting device. The detachable connection device is for example a plug-turn Connection as described in EP 2 745 965 A1 or EP 2 745 966 A1. The design of the drill shaft as a one-piece or two-piece drill shaft has no influence on the structure of the drill segments 11A or the drill ring 11B.

FIGN. 2A, B show two variants of a machining tool designed as a saw blade 20A, 20B. The in FIG. 2A is shown as the first saw blade and the one shown in FIG. The saw blade 20B shown in FIG. 2B is referred to as the second saw blade, and the first and second saw blades 20A, 20B are also summarized under the term “saw blade”.

The first saw blade 20A comprises a plurality of machining segments 21A, a disk-shaped base body 22A and a tool holder. The machining segments 21A which are used for sawing are also referred to as saw segments and the disk-shaped base body 22A is also referred to as the master blade. The saw segments 21A are firmly connected to the master blade 22A, for example by screwing, gluing, soldering or welding.

The second saw blade 20B comprises a plurality of machining segments 21B, an annular base body 22B and a tool holder. The processing segments 21 B, which are used for sawing, are also referred to as saw segments and the ring-shaped base body 22B is also referred to as a ring. The saw segments 21B are firmly connected to the ring 22B, for example by screwing, gluing, soldering or welding.

The saw blade 20A, 20B is connected to a saw via the tool holder and is driven by the saw in a direction of rotation 24 about an axis of rotation 25 in the sawing operation. During the rotation of the saw blade 20A, 20B about the axis of rotation 25, the saw blade 20A, 20B is moved along a feed direction, the feed direction being parallel to the longitudinal plane of the saw blade 20A, 20B. The saw blade 20A, 20B creates a saw slot in the workpiece to be machined.

FIG. 3 shows a machining tool designed as a removal disk 30. The removal disc 30 comprises a plurality of machining segments 31, a base body 32 and a tool holder. The processing segments 31, which are used for removal, are also referred to as removal segments and the disk-shaped basic body 32 is also referred to as a pot. The removal segments 31 are firmly connected to the pot 32, for example by screwing, gluing, soldering or welding.

The removal disk 30 is connected via the tool holder to a tool device and in the removal mode from the tool device in a direction of rotation 34 about an axis of rotation 35 driven. During the rotation of the removal disk 30 about the axis of rotation 35, the removal disk 30 is moved over a workpiece to be machined, the movement being perpendicular to the axis of rotation 35. The removal disk 30 removes the surface of the workpiece to be machined.

FIG. 4 shows a processing tool designed as a cut-off chain 36. The abrasive chain 36 comprises a plurality of processing segments 37, a plurality of link-shaped base bodies 38 and a plurality of connecting members 39. The processing segments 37 which are used for cut-off grinding are also referred to as cut-off segments and the link-shaped base body 38 are also referred to as drive links.

The drive links 38 are connected via the connecting links 39. In the game Ausführungsbei the links 39 are connected to the drive links 38 via rivet bolts. The rivet bolts allow rotation of the drive links 38 relative to the connecting links 39 about an axis of rotation which runs through the center of the rivet bolts. The machining segments 37 are firmly connected to the drive members 38, for example by screwing, gluing, soldering or welding.

The cut-off chain 36 is connected via a tool holder to a tool device and is driven in operation by the tool device in one direction of rotation. During the rotation of the cut-off chain 36, the cut-off chain 36 is moved into a workpiece to be machined.

FIGN. 5A-C show a machining segment 41 in a three-dimensional representation (FIG. 5A), in a view on an upper side of the machining segment 41 (FIG. 5B) and in a view on a side surface of the machining segment 41 (FIG. 5C).

The processing segment 41 corresponds in structure and composition to the processing segments 11A, 21 A, 21 B, 31, 37; the machining segment 11 B formed as a drilling ring differs from its machining segment 41 by its annular structure. The machining segments can differ from one another in the dimensions and in the curvatures of the surfaces. The basic structure of the machining segments according to the invention is explained on the basis of the machining segment 41 and applies to the machining segments 11 A, 11 B of FIGN. 1A, B, for the processing segments 21 A, 21 B of FIGN. 2A, B, for the machining segment 31 of FIG. 3 and for the machining segment 37 of FIG. 4th

The processing segment 41 is constructed from a processing zone 42 and a neutral zone 43. The neutral zone 43 is required if the machining segment 41 is to be connected to the base body of a machining tool; with processing segments elements that are connected to the base body, for example by soldering or gluing, the neutral zone 43 can be omitted. The processing zone 42 is constructed from a first matrix material 44 and first hard material particles 45, and the neutral zone 43 is constructed from a second matrix material 46 without hard material particles.

The term "hard material particles" summarizes all cutting agents for processing segments; These include, in particular, individual hard material particles, composite parts made of several hard material particles and coated or encapsulated hard material particles. The term "matrix material" summarizes all materials for the construction of machining segments in which hard material particles can be embedded. Matrix materials can consist of one material or be composed of a mixture of different materials.

Machining segments that are produced with the method according to the invention for producing a machining segment have a layer with first hard material particles 45, further layers with first hard material particles 45 are not provided. The "first hard material particles" refer to the hard material particles of the machining segment 41 which, after the machining segment has been produced, have an overhang on the upper side of the first matrix material 44. Hard material particles that are completely embedded in the first matrix material 44 in the processing segment 41 do not fall under the definition of the first hard material particles.

The processing segment 41 is connected to an underside 47 with the main body of the machining tool. In the case of machining segments for core drilling and machining segments for removal, the underside of the machining segments is generally flat, whereas the underside of machining segments for sawing has a curvature in order to be able to fasten the machining segments to the curved end face of the annular or disk-shaped base body.

The first hard material particles 45 are arranged according to a defined particle pattern in the first matrix material 44 (FIG. 5B) and have a projection Ti on an upper side 48 of the machining segment 41 opposite the lower side 47, compared to the first matrix material 44. In the embodiment of FIGN. 5A-C, the processing segment 41 comprises a number of 9 first hard material particles 45 which protrude on the top 48. The number of first hard material particles 45 and the defined particle pattern in which the first hard material particles 45 are arranged in the first matrix material 44 are adapted to the requirements of the machining segment 41. The first hard material particles 45 generally originate from a particle distribution which is characterized by a minimum diameter, a maximum diameter and an average diameter. Due to the particle distribution of the first hard material particles 45 between the minimum and maximum diameter, the protrusions of the first hard material particles 45 can vary accordingly. In the exemplary embodiment, all of the first hard material particles 45 have a protrusion of more than 400 mhi compared to the surrounding first matrix material 44.

The in the FIGN. 1A, B, FIGN. 2A, B, FIG. 3 and FIG. 4 shown Bear processing tools according to the invention, which are provided for the processing of concrete materials, have a defined direction of rotation. When viewed in the direction of rotation of the machining tool, a distinction can be made between a front area and a rear area of a hard material particle 45. Due to its geometry with a flat underside, the machining segment 41 is suitable as a drilling segment for the core drill bit 10A.

The direction of rotation 14 of the core drill bit 10A defines a front area 51 and a rear area 52. The processing of concrete materials takes place in the front areas 51 of the first hard material particles 45 and the processing rate depends essentially on the size of the protrusion of the first hard material particles in the front area Chen 51. The first hard material particles 45 have a front projection T _front in the front area 51 and a rear projection T _back in the rear area, which correspond in the exemplary embodiment. Alternatively, the first hard material particles 45 can have different protrusions T _{fr0nt on the front} and protrusions T _back on the rear.

The processing segment 41 is produced using the method according to the invention in three stages: in a first stage, a green compact 53 is produced, in a second stage the green compact 53 is compressed into a compact 54 and in a third stage the compact 54 becomes the processing segment 41 processed further. FIG. 6 shows the green compact 53 and the compact 54. The green compact 53 is constructed from the first matrix material 44 and the first hard material particles 45, and a top layer 55 of a matrix material is additionally applied. The first matrix material 44 or a second matrix material 56, which is different from the first matrix material 44, is suitable as the matrix material for the cover layer.

The green compact 53 is compressed under the action of pressure until the compact 54 has essentially the final geometry of the machining segment 41. Cold pressing processes or hot pressing processes are suitable, for example, as processes which achieve a pressure effect on the green compact 53. In cold pressing processes, the green compact 53 is only exposed to pressure, while the green compact 53 in hot pressing processes is exposed not only to the pressure, but also to temperatures of up to approx. 200 ° C. The compact 54 is further processed under the influence of temperature, for example during sintering or by infiltration, to form the processing segment 41. FIGN. 7A-C show some tool components that are used in the manufacture of the machining segment 41 using the method according to the invention. The tool components include a lower stamp 61, a die 62 and an upper stamp 63, the lower stamp 61 also being the first press stamp and the upper stamp

63 is referred to as the second press stamp. FIGN. 7B and 7C show the upper stamp 63 in detail.

The green body 53 is built up in the die 62 with a cross-sectional area that corresponds to the desired geometry of the green body 53. The die 62 has a first opening on the underside, into which the lower punch 61 is displaceable, and on the upper side a second opening, into which the upper punch 63 can be displaced. The upper punch 63 has depressions 64 in the pressing surface, the arrangement of which corresponds to the defined particle pattern of the first hard material particles 45.

The green compact 53 is built up from the bottom. The first matrix material 44 is filled into the die 62 with the aid of a filling shoe until the desired filling height is reached. The first hard material particles 45 are placed in the first matrix material 44 in accordance with the defined particle pattern in the surface of the first matrix material 44 and embedded in the first matrix material 44 up to a desired embedding depth. The finished green ling 53 is compressed under pressure using the lower ram 61 and the upper ram 63 to the compact 54. The compacting of the green compact 53 to form the compact 54 takes place with the help of the special upper punch 63 in a pressing direction perpendicular to the cross-sectional area of the green compact 53. The depressions 64 in the pressing surface of the upper punch 63 have an arrangement which corresponds to the defined particle pattern of the first hard material particles 45. With the help of the special upper punch 63, the processing segments 41 can be produced who are suitable for the dry processing of concrete materials. The wells

64 are required to produce the protrusion of the first hard material particles 45 on the top 48 during pressing.

With direct contact between the first hard material particles 45 and the depressions 64 of the upper punch 63, increased wear of the upper punch 63 can occur. In order to reduce the wear of the upper punch 63, direct contact of the first hard material particles 45 with the upper punch 63 should be avoided. Suitable measures are the application of a matrix material as a top layer and / or the use of enveloped first hard material particles 45.

After the placement of the first hard material particles 45, a cover layer 55 of the first matrix material 45 can be applied. Alternatively, a top layer 55 of the second matrix material 56 can be applied, the second matrix material 56 being made of the first matrix material. rix material 44 is different. When using a second matrix material 56, which is different from the first matrix material 44, matrix materials with different wear properties can be used. The second matrix material 56 serves to protect the upper punch 63 when compacting the green compact 53 and should be able to be removed as quickly as possible in the finished machining segment in order to expose the first hard material particles 45 which process the substrate. A second matrix material 56 with a higher wear rate than the first matrix material 44 can be removed quickly.

The use of coated first hard material particles has the advantage that the first hard material particles 45 do not come into direct contact with the upper punch 63 and the wear of the upper punch 63 can be reduced. The first matrix material 44 can be used as the shell material for the first hard material particles 45. Alternatively, a second matrix material can be used as the shell material for the first hard material particles 45, the second matrix material being different from the first matrix material 44. When using a cladding material that is different from the first matrix material 44, matrix materials with different wear properties can be used. The shell material serves to protect the upper punch 63 during compaction and should be able to be removed as quickly as possible in the finished machining segment in order to expose the first hard material particles 45 which process the concrete material.

Depending on the wear properties of the first matrix material 44, increased wear of the first matrix material 44 on the side surfaces of the processing segment can occur during the processing of a substrate with the processing segment 41 due to friction with the substrate. This wear can be reduced by means of second hard material particles. The second hard material particles can be admixed to the first matrix material 44 as statistically distributed particles, or the second hard material particles are placed in the first matrix material 44 according to a defined second particle pattern. The second hard material particles are placed in particular in the area of the side surfaces of the processing segment 41.

Claims

1. Method for producing a machining segment (11A, 11 B; 21 A, 21 B; 31; 37;

41) for a machining tool (10A, 10B; 20A, 20B; 30; 36), the machining segment having an underside (47) with a base body (12A, 12B; 22A, 22B; 32; 38) of the machining tool (10A, 10B; 20A, 20B; 30; 36) is connected with the steps:

^■ a green compact (53) is composed of a first matrix material (44) and the first hard material particles constructed (45), wherein the first hard material particles (45) are placed according to a de-defined particle pattern in the first matrix material (45),

^■ The green compact (53) is under pressure between a first press stamp (61), which forms the underside (47) of the machining segment, and a second press stamp (63), which is an underside (47) opposite the upper side (48) of the Machining segment forms, compacted into a compact (54) and

^■ the pellet (54) under the action of temperature or by infiltrating the edit segment (41) further processed,

characterized in that when compacting the green compact (53) a second press ram (63) is used, which has depressions (64) in a pressing surface, the arrangement of the depressions (64) corresponding to the defined particle pattern of the first hard material particles (45).

2. The method according to claim 1, characterized in that after the placement of the first hard material particles (45) a top layer (55) of the first matrix material (44) is applied.

3. The method according to claim 1, characterized in that after the placement of the first hard material particles (45) a cover layer (55) of a second matrix material (56) is applied, the second matrix material (56) from the first matrix material (44) ver is different.

4. The method according to claim 1, characterized in that after the placement of the first hard material particles (45) a first cover layer of the first matrix material (44) and a second cover layer of a second matrix material (56) is applied, the second matrix material (56) is different from the first matrix material (44).

5. The method according to any one of claims 1 to 4, characterized in that first hard material particles (45) are used which are enveloped by a shell material, wherein the shell material corresponds to the first matrix material (44).

6. The method according to any one of claims 1 to 4, characterized in that first hard material particles (45) are used which are enveloped by a shell material, wherein the shell material is different from the first matrix material (44).

7. The method according to any one of claims 1 to 6, characterized in that the first matrix material (44) second hard material particles are admixed, wherein an average particle diameter of the second hard material particles is smaller than an average particle diameter of the first hard material particles (45).